EPR signals of Fe(III) bleomycin at 77 and 15 K in Hepes or phosphate buffers at pH 7.4 were obtained. The ratio of high spin to low spin in Hepes was 60:40, while that observed in phosphate buffer was 80:20. Addition of excess cyanide gave a new high spin signal. Addition of DNA to CNFe(III)Blm changed the high spin signal to a low spin signal, which is indicative of the release of cyanide.